Add Two Numbers
Recognize the pattern
Brute force idea
If you approach Add Two Numbers in the most literal way possible, you get this: Convert to numbers, add, convert back — fails for very large numbers. It is a fair place to begin because it matches the surface of the question, yet it does not capture the deeper structure that makes the problem simpler.
Better approach
A calmer way to see Add Two Numbers is this: Traverse both lists simultaneously, add digits with carry. The goal is not to be clever for its own sake, but to remember the one relationship that keeps the solution grounded as you move forward.
Key invariant
The truth you want to protect throughout Add Two Numbers is this: Process one digit at a time, propagating carry to the next position. If that remains true after every update, the rest of the reasoning has a stable place to stand.
Watch out for
A common way to get lost in Add Two Numbers is this: Forgetting the final carry — if the last addition produces a carry, add an extra node. Most mistakes here are not about syntax; they come from losing track of what your state, pointer, or structure is supposed to mean.